Long term analyte sensor array

ABSTRACT

A long term analyte sensor for measuring at least one analyte in the body of a user and which includes a housing, a plurality of analyte contacting sensor elements and at least one structure for relaying information away from the sensor. This plurality of analyte contacting sensor elements are typically disposed in an array. The analyte sensor further includes at least one sensor protection membrane that is controllable in a manner such that sensor elements may be activated (e.g. exposed to the external environment) at different times so as to extend the useful life of the sensor. In illustrative analyte sensors, the analyte is glucose.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending andcommonly-assigned U.S. patent application Ser. No. 10/989,038, filedNov. 15, 2004, which claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional patent application Ser. No. 60/519,709, filed on Nov. 13,2003, the contents of which are incorporated by reference.

This application is related to U.S. patent application Ser. No.10/273,767 filed Oct. 18, 2002 (published as US-2004-0074785-A1) andU.S. patent application Ser. No. 10/861,837, filed Jun. 4, 2004, thecontents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to analyte sensors for long termuse. In certain embodiments, the analyte sensor is for measuring glucoseand includes multiple elements that can be replaced or used as otherelements are depleted or fail to operate. This expands the longevity ofthe sensors.

2. Description of Related Art

The assay of biochemical analytes such as glucose and lactate isimportant in a variety of clinical contexts. For example, the monitoringof glucose concentrations in fluids of the human body is of particularrelevance to diabetes management. Continuously or intermittentlyoperating glucose sensors, including sensors implanted in the humanbody, are sought for the management of diabetes, for example, forwarning of imminent or actual hypoglycemia as well as its avoidance. Themonitoring of lactate concentrations in fluids of the human body isuseful in, but not limited to, the diagnosis and assessment of a numberof medical conditions including trauma, myocardial infarction,congestive heart failure, pulmonary edema and septicemia.

Biomedical measuring devices commonly used to monitor physiologicalvariables include amperometric sensor devices that utilize electrodesmodified with an appropriate enzyme coating. Sensors having such enzymeelectrodes enable the user to determine the concentration of variousanalytes rapidly and with considerable accuracy, for example byutilizing the reaction of an enzyme and an analyte where this reactionutilizes a detectable coreactant and/or produces a detectable reactionproduct. For example, a number of glucose sensors have been developedthat are based on the reaction between glucose and oxygen that iscatalyzed by glucose oxidase (GOx) as shown in FIG. 1. In this context,the accurate measurement of physiological glucose concentrations usingsensors known in the art, typically requires that both oxygen and waterbe present in excess. As glucose and oxygen diffuse into an immobilizedenzyme layer on a sensor, the glucose reacts with oxygen to produceH₂O₂. Glucose can be detected electrochemically using the immobilizedenzyme glucose oxidase coupled to oxygen and/or hydrogenperoxide-sensitive electrodes. The reaction results in a reduction inoxygen and the production of hydrogen peroxide proportional to theconcentration of glucose in the sample medium. A typical device iscomposed of (but not limited to) at least two detecting electrodes, orat least one detecting electrode and a reference signal source, to sensethe concentration of oxygen or hydrogen peroxide in the presence andabsence of enzyme reaction. Additionally, the complete monitoring systemtypically contains an electronic sensing and control apparatus fordetermining the difference in the concentration of the substances ofinterest. From this difference, the concentration of analytes such asglucose can be determined.

A wide variety of such analyte sensors as well as methods for making andusing such sensors are known in the art. Examples of such sensors,sensor sets and methods for their production are described, for example,in U.S. Pat. Nos. 5,390,691, 5,391,250, 5,482,473, 5,299,571, 5,568,806as well as PCT International Publication Numbers WO 01/58348, WO03/034902, WO 03/035117, WO 03/035891, WO 03/023388, WO 03/022128, WO03/022352, WO 03/023708, WO 03/036255, WO03/036310 and WO 03/074107, thecontents of each of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

Embodiments of the invention disclosed herein provide long term analytesensors of the type used, for example, in subcutaneous or transcutaneousmonitoring of blood glucose levels in a diabetic patient. Embodiments ofthe invention disclosed herein further provide analyte sensors of thetype used, for example, in a variety of clinical contexts such as withdialysis and/or extracorporeal membrane oxygenation protocols. Morespecifically, the disclosure provided herein teaches optimized long termanalyte sensor designs and methods for making and using such sensors.

An illustrative embodiment of the present invention is a long termanalyte sensor for measuring at least one analyte in the body of a userand which includes a housing, a plurality of analyte sensor elements andat least one structure for relaying information away from the sensor.This plurality of analyte sensor elements are typically disposed in anarray. The analyte sensor further includes at least one sensorprotection membrane that is controllable in a manner such that one ormore of the plurality of analyte sensor elements may be activated (e.g.exposed to analyte) at different times so as to extend the useful lifeof the sensor. In alternative embodiments, one or more of the pluralityof analyte sensor elements may allow exposure without producing anelectrical current until that element is selected to be electricallyactive.

Another illustrative embodiment of the invention is an analyte sensingdevice for sensing at least one analyte, the analyte sensing devicecomprising: a plurality of analyte sensor elements adapted to contactand sense analyte; at least one analyte sensor membrane disposed upon atleast one of the plurality of analyte sensor elements in a manner thatreversibly prevents an analyte from contacting the at least one of theplurality of analyte sensor elements, wherein the permeability of theanalyte sensor membrane can be controlled to allow an analyte to contactat least one of the plurality of analyte sensor elements; and at leastone structure operatively coupled to the analyte sensing device forrelaying information away from the analyte sensing device. Optionally,the plurality of analyte sensor elements that contact and sense theanalyte are disposed in an array in the analyte sensing device. In suchdevices, the permeability of the analyte sensor membrane is typicallycontrolled so that a second analyte sensor element in the plurality ofanalyte sensor elements contacts analyte after a first analyte sensorelement in the plurality of analyte sensor elements exhibits a decreasein the ability to sense analyte due to biofouling and/or loss ofactivity of an analyte sensing enzyme disposed in the first analytesensor element, so that the useful life of the analyte sensing device isextended. In certain embodiments of the invention, the analyte sensingdevice is implantable within the body of a mammal. In particularembodiments, the analyte is glucose. In alternative embodiments, theanalyte is a protein, lactose, a carbohydrate, a saccharide, a mineral,and element, a small molecule compound, a virus, a peptide, a proteinfragment, an analogue of a compound, a medication, a drug, an element ofa body chemistry assay, body constituent or byproduct, or the like.

As discussed in detail below, the analyte sensor membrane can be madeusing a number of different methods and materials know in the art. Forexample, in one embodiment, the analyte sensor membrane comprises arupturable metallic membrane that hermetically seals the analyte sensorelement. Alternatively, the analyte sensor membrane comprises abiodegradable polymer that degrades at a defined rate within an in vivoenvironment. In certain embodiments of the invention, the analyte sensormembranes and/or the analyte sensing elements are discreetly controlledto allow rupture of a specific membrane and/or interrogation and receiptof signal from a specific analyte sensing element. Optionally, at leastone of the analyte sensor elements in the analyte sensing devicecomprises a hydrogel disposed thereon, wherein upon exposure to anaqueous solution, the hydrogel expands in a manner that increases thepermeability of the analyte sensor membrane.

Another embodiment of the invention is a method of making a sensorapparatus for implantation within a mammal comprising the steps of:providing a plurality of analyte sensor elements adapted to contact andsense analyte; providing at least one analyte sensor membrane disposedupon at least one of the plurality of analyte sensor elements in amanner that reversibly prevents an analyte from contacting the at leastone of the plurality of analyte sensor elements, wherein thepermeability of the analyte sensor membrane can be controlled to allowan analyte to contact at least one of the plurality of analyte sensorelements; and providing at least one structure operatively coupled tothe analyte sensing device for relaying information away from theanalyte sensing device.

Another embodiment of the invention is a method of sensing an analytewithin the body of a mammal, the method comprising implanting an analytesensor in to the mammal, the analyte sensor comprising: a plurality ofanalyte sensor elements adapted to contact and sense analyte; at leastone analyte sensor membrane disposed upon at least one of the pluralityof analyte sensor elements in a manner that reversibly prevents ananalyte from contacting the at least one of the plurality of analytesensor elements, wherein the permeability of the analyte sensor membranecan be controlled to allow an analyte to contact at least one of theplurality of analyte sensor elements; and at least one structureoperatively coupled to the analyte sensing device for relayinginformation away from the analyte sensing device; and sensing an analytewithin the body of a mammal.

Yet another embodiment of the invention is a method of extending theuseful life of an analyte sensing device comprising analyte sensorelements that exhibit a decrease in the ability to sense analyte overtime due to biofouling or a loss of activity of an analyte sensingenzyme disposed on an analyte sensor element; the method comprisingsensing an analyte with an analyte sensing device comprising: aplurality of analyte sensor elements adapted to contact and senseanalyte; at least one analyte sensor membrane disposed upon at least oneof the plurality of analyte sensor elements in a manner that reversiblyprevents an analyte from contacting the at least one of the plurality ofanalyte sensor elements, wherein the permeability of the analyte sensormembrane can be controlled to allow an analyte to contact at least oneof the plurality of analyte sensor elements; and at least one structureoperatively coupled to the analyte sensing device for relayinginformation away from the analyte sensing device; wherein the usefullife of an analyte sensing device is extended by: deactivating a firstanalyte sensor element in the plurality of analyte sensor elements thatcontact and sense analyte when the first analyte sensing elementexhibits a decrease in the ability to sense analyte due to biofouling ora loss of activity of an analyte sensing enzyme disposed on the firstanalyte sensor element; and activating a second analyte sensor elementin the plurality of analyte sensor elements adapted to contact and senseanalyte by controlling the permeability of an analyte sensor membranedisposed upon the second analyte sensor element to allow an analyte tocontact the second analyte sensor element, so that the useful life ofthe analyte sensing device is extended.

Embodiments of the invention also provide additional articles ofmanufacture including sensor elements, sensor sets and kits. In one suchembodiment of the invention, a kit and/or sensor element or set, usefulfor the sensing an analyte as is described above, is provided. The kitand/or sensor set typically comprises a container, a label and a sensoras described above. The typical embodiment is a kit comprising acontainer and, within the container, an analyte sensor apparatus havinga design as disclosed herein and instructions for using the analytesensor apparatus.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription. It is to be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the present invention are given by way of illustrationand not limitation. Many changes and modifications within the scope ofthe present invention may be made without departing from the spiritthereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a schematic of the well known reaction between glucoseand glucose oxidase. As shown in a stepwise manner, this reactioninvolves glucose oxidase (GOx), glucose and oxygen in water. In thereductive half of the reaction, two protons and electrons aretransferred from β-D-glucose to the enzyme yielding d-gluconolactone. Inthe oxidative half of the reaction, the enzyme is oxidized by molecularoxygen yielding hydrogen peroxide. The d-gluconolactone then reacts withwater to hydrolyze the lactone ring and produce gluconic acid. Incertain electrochemical sensors of the invention, the hydrogen peroxideproduced by this reaction is oxidized at the working electrode(H₂O₂→2H++O₂+2e⁻).

FIG. 2 provides a diagrammatic view of a typical analyte sensing elementconfiguration of an embodiment of the current invention.

FIG. 3 provides a diagram of a glucose sensor array showingenzyme/membrane array with electronics adhered to electrode array withelectronics and lead connections.

FIG. 4 provides a diagram of a working electrode array with electronicshoused under hermetic lid. Power and information is transferred throughthe lead connection pads to the electronics.

FIG. 5 provides a diagram of an array containing wells filled withenzyme and covered with dissolvable membrane. Electronics for individualaddressing are contained under the hermetic lid. Information and powerare transferred from the electrode array through hermetic vias.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless otherwise defined, all terms of art, notations and otherscientific terms or terminology used herein are intended to have themeanings commonly understood by those of skill in the art to which thisinvention pertains. In some cases, terms with commonly understoodmeanings are defined herein for clarity and/or for ready reference, andthe inclusion of such definitions herein should not necessarily beconstrued to represent a substantial difference over what is generallyunderstood in the art. Many of the techniques and procedures describedor referenced herein are well understood and commonly employed usingconventional methodology by those skilled in the art. As appropriate,procedures involving the use of commercially available kits and reagentsare generally carried out in accordance with manufacturer definedprotocols and/or parameters unless otherwise noted.

Embodiments of the invention disclosed herein provide analyte sensingdevices having enhanced material properties such as extended usefullifetimes. The disclosure further provides methods for making and usingsuch sensors. While particular embodiments of the invention pertain toglucose and/or lactate sensors, a variety of the elements disclosedherein (e.g. analyte sensor membranes) can be adapted for use with anyone of the wide variety of sensors known in the art. The analyte sensorelements, architectures and methods for making and using these elementsthat are disclosed herein can be used to establish a variety of sensorstructures. Such sensor embodiments of the invention exhibit asurprising degree of flexibility and versatility, characteristic whichallow a wide variety of sensor configurations to be designed to examinea wide variety of analyte species.

In typical embodiments of the present invention, the transduction of theanalyte concentration into a processable signal is by electrochemicalmeans. These transducers may include any of a wide variety ofamperometric, potentiometric, or conductimetric base sensors known inthe art. Moreover, the microfabrication sensor techniques and materialsof the instant invention may be applied to other types of transducers(e.g., acoustic wave sensing devices, thermistors, gas-sensingelectrodes, field-effect transistors, optical and evanescent field waveguides, and the like) fabricated in a substantially nonplanar, oralternatively, a substantially planar manner. A useful discussion andtabulation of transducers which may be exploited in a biosensor as wellas the kinds of analytical applications in which each type of transduceror biosensor, in general, may be utilized is found in an article byChristopher R. Lowe in Trends in Biotech. 1984, 2 (3), 59-65.

Specific aspects of embodiments of the invention are discussed in detailin the following sections.

I. Typical Elements, Configurations and Analyte Sensor Embodiments ofthe Invention

A. Sensor Protection Membrane Embodiments of the Invention

Long-term (e.g. “prolonged” and “permanent” sensors) analyte sensorssuch as glucose sensors must operate reliably in harsh environments(e.g. the body) and are often subject to loss of sensitivity for avariety of reasons. These reasons include but are not limited tobio-fouling, loss of enzyme activity due to both environmental andfundamental chemical processes, increases in mass transfer coefficientsand macrophage encapsulation. In addition, implanted enzymatic sensors,particularly glucose sensors, need to have a significant amount ofenzyme for long-term survival. The difficulty with these implantedsensors is that the large mass of enzyme necessarily acts as a masstransfer barrier, thus increasing the response time.

Embodiments of the invention disclosed herein is designed to addresssuch limitations by teaching analyte sensors having a plurality ofsensing elements that are covered by one or more controllable sensorprotection membranes. In particular, embodiments of the sensorsdisclosed herein incorporate one or more sensor protection membranesthat are controllable in a manner such that one or more of the pluralityof sensor elements may be activated (e.g. exposed to the externalenvironment) at different times during the life of the analyte sensor soas to extend the useful life of the sensor. The analyte sensingprotection membranes can take a variety of structural forms (e.g. afilm, a layer, a cap etc.) as long as they function to reversiblyprotect the analyte sensing element of the analyte sensing device fromthe environment into which the analyte sensing device is placed.Embodiments of the invention disclosed herein further include methodsfor making the sensors of the invention. The following paragraphs ofthis section provide a description of typical embodiments of theinvention.

One embodiment of the invention is a single chip based sensor thatcontains a series of individual sensors with limited life-time (weeks tomonths) that are initially stored inside a hermetically sealed chamberand which can be addressed individually on-demand. In this embodiment ofthe invention, certain aspects of the sensor are similar to devices usedin drug delivery technologies known in the art (see, e.g. U.S. Pat. Nos.6,551,838, 6,491,666, 6,527,762, U.S. Patent Application No. 20040106914and Santini, et al. Nature 397, 28 Jan. 1999, the contents of each ofwhich are incorporated by reference). Briefly, in this drug deliverytechnology, a chip is constructed which contains a large number ofreservoirs, each containing a drug. A barrier such as a gold foilmembrane covers each reservoir to produce a sealed compartment. When analiquot of drug is desired, an electrical pulse can delivered to one ormore of the foil membrane(s) which results in the drug eluting out ofthe compartment. In addition, certain embodiments of the invention aresimilar to serial sensor technologies known in the art and which aredescribed for example in U.S. Pat. No. 5,999,848 which is incorporatedherein by reference.

Embodiments of the invention include an analyte sensing device having aplurality of analyte sensor elements that are covered by a barriermembrane (e.g. an analyte sensor membrane). In some embodiments of theinvention, the barrier membrane creates a hermetic seal over the analytesensor element. Certain embodiments of the analyte sensor device providea long-term implantable sensor with improved characteristics is obtained(e.g. improved mass transfer characteristics). When the analyte sensormembrane covering a particular sensor is controllably permeabilized in amanner similar to that described above, that analyte sensor element thenbecomes “active” and provides input to an analyte sensing device (e.g.an implanted or an external device), whose performance can be modifiedby the parameter in question. Should this activated analyte sensorelement become unstable or ineffective due to any of a number of reasons(e.g. biofouling), it can be electronically inactivated and anothersensor on the analyte sensing device can be activated. Electroniccontrols for the analyte sensor device can for example incorporate bothswitching circuits and common electrodes for both reference and counterelectrodes.

Applications of embodiments of this invention include continuous sensingof glucose in instances where the analyte sensing element has a limitedperformance lifetime in the body. In another embodiment of theinvention, the analyte sensing device can have a number of sensorplatforms (glucose, lactate, pH, oxygen) and different sensors can beactivated depending on the medical condition of the patient asdetermined by some set of existing sensors. For example, in a criticalcare environment a patient might be monitored for glucose and lactateusing a multianalyte sensing device embodiment of the invention. If theglucose signal is stable and the lactate sensor shows an increase inlactate, then pH and O₂ analyte sensing elements can be activated tomonitor for sepsis. Similarly, analyte sensing elements that are notstable enough for long-term use can be activated only when necessary.Another embodiment of this technology includes monitoring for viralinfection (hepatitis, HIV etc.) or cancer during the course of therapy,i.e. one year. In particular, such an analyte sensing device makesdiscrete counts of viral load (or cancer chemokines or others) on aregular basis. Optionally the analyte sensing device can be implantednear a tumor site or in the liver (for hepatitis) and accessedperiodically via external interrogation without the need for concomitantsurgeries or invasive tissue testing.

Another embodiment of the invention provides sensor array of analytesensing elements, optionally within reservoirs/wells and sealed withcontrollable membranes and which is useful for long term analytesensing. An illustrative sensor array consists of at least 24 wells in adielectric skeleton (either patterned with ion beam assisted deposition(IBAD) alumina or drilled into a ceramic substrate), with each sensorelement lifetime spanning 2-4 weeks. The base of the well (on a baseceramic with the patterned IBAD alumina wells, or a separate ceramicsubstrate soldered to the drilled substrate wells) can have a metallizedworking electrode covered by an immobilized enzyme such as glucoseoxidase. The glucose oxidase can be covered by a material such as aGlucose Limiting Membrane (GLM) within the well or on top of themembrane. In a specific example, the well can be hermetically sealedwith a gold membrane until programmed voltage-induced dissolution of themembrane. Alternatively, the analyte sensing element can be coated withan expanding hydrogel within the well, such that the voltage-induceddissolution of a portion of the gold membrane induces expansion of thehydrogel, thus mechanically assisting the removal of the membrane fromthe well's surface. Once the contents of the well are exposed, a workingelectrode in the analyte sensing element can be individuallyinterrogated. The individual interrogation allows focused sensorreadings, while isolating spent sensor elements from obscuring the newlyexposed sensor signal. The counter and reference electrodes necessaryfor electrochemical sensor function may be common to the entire array,or located within each well.

Addressing of membranes and electrodes in the analyte sensing devices ofthe invention may be achieved by individual signal traces to eachposition, or in a similar manner to that used in active matrix displaytechnology. Active matrix addressing utilizes a grid pattern with eachaddressable position situated at the nodal point. Activation of theappropriate row and column traces will trigger the desired nodalfunction (electrode reading or membrane dissolution). Addressing ofspecific traces can be achieved by an integrated circuit, masterpotentiostat, and a series of programmable digital switches, possiblyutilizing hermetic sealing and via technology. Alternatively, theelectronics can be packaged at some distant location on the sensorassembly, or separated from the circuitry on an implant unit as is knownin the art. Optionally, an analyte sensing device can be programmed toinitiate the disintegration or permeabilization of the analyte sensorprotective membrane in response to a variety of conditions, including aspecific decrease in the function of an active analyte sensor element(e.g. a defined and/or predetermined decrease in function due tobiofouling and/or enzyme inactivation) a specific time period, receiptof a signal from another device (for example by remote control orwireless methods), or detection of a particular condition in theenvironment in which the sensor is placed (e.g. an increase in lactateconcentration) Such sensor arrays provide a long term glucose sensorwith the dynamic properties of a short term sensor. FIGS. 3-5 provideillustrative embodiments of a sensor array (e.g. a glucose sensor array)with addressable components.

In certain embodiments of the invention, the analyte sensor membrane canbe a material that is permeabilizable in response to an applied signalsuch as an electric field or current, magnetic field, change in pH, orby thermal, chemical, electrochemical, or mechanical signal. Optionally,the analyte sensor membrane can be a rupturable thin metal (e.g., gold)membrane and can be impermeable to the surrounding environment (e.g.,body fluids or another chloride containing solution). Based on the typeof metal and the surrounding environment, a particular electricpotential can be applied to this metal analyte sensor membrane. Themetal analyte sensor membrane can then oxidize and dissolve by anelectrochemical reaction, “exposing” the contents of the reservoir tothe surrounding environment. In addition, materials that normally forminsoluble ions or oxidation products in response to an electricpotential can be used if, for example, local pH changes near the anodecause these oxidation products to become soluble. Examples of suitableanalyte sensor membrane materials include metals such as copper, gold,silver, and zinc, and some polymers known in the art. In anotherembodiment, the analyte sensor membrane can be a polymer with a specificmelting point above body temperature. When the local temperature nearthe polymer analyte sensor membrane is increased above the polymer'smelting point, for example using thin film resistors located near theanalyte sensor membrane, the analyte sensor membrane melts and exposesthe analyte sensing element to the surrounding environment.

The specific properties of the analyte sensor membrane can be selectedbased on a variety of factors such as the period over which exposure ofthe analyte sensing element is desired, generally in the range of weeksto months. In some in vivo embodiments, a single analyte sensing devicehaving a plurality of analyte sensing elements sensors can have theplurality of sensing elements activated sequentially. In this context,by sequentially activating a new sensor as the previously activatedsensor loses its ability to sense analyte allows the analyte sensingdevice to sense analytes for an extended period of time, for example oneto twelve months.

In certain embodiments of the invention, the analyte sensor membrane canbe made from a material that degrades at a defined rate in an in vitroand/or in vivo environment so that the analyte sensing element isexposed to the analyte upon degradation of this material. A number ofsuch polymers are known in the art and are generally termedbiodegradable and/or bioerodable. In this context, at least two types ofdegradation can occur with such polymers. One type of degradation isbulk degradation, in which the polymer degrades in a fairly uniformmanner throughout the matrix. The prevailing mechanism of bulkdegradation is hydrolysis of the hydrolytically unstable polymerbackbone. First, water penetrates the bulk of the solid polymericimplant, preferentially attacking chemical bonds in the amorphous phaseand converting long polymer chains into shorter water-soluble fragments.This results, initially, in a reduction in molecular weight (M_(n))without an immediate change in physical properties. A second type ofdegradation is surface erosion, typically called bioerosion. Bioerosioncan occur when the rate at which water penetrates the coating of theimplant is slower than the rate of the conversion of the polymer intowater-soluble materials.

Commonly used biodegradable polymers are typically of thepoly(hydroxyacid) type, in particular poly(L-lactic acid),poly(D,L-lactic acid), poly(glycolic acid), and copolymers thereof. Atypical copolymer is poly(lactide-co-glycolide), abbreviated as PLGA.These materials are broken down in the body to the non-toxic productslactic acid and glycolic acid, and have been approved by the Food andDrug Administration for use as resorbable sutures, in bone implants, andas controlled release microspheres. Other polymers being utilizedinclude poly(funimaric anhydride) and poly(sebacic anhydride).Mathiowitz, E., Jacob, J. S., Jong, Y. S., Carino, G. P., Chickering, D.E., Chaturvedi, P., Santos, C. A., Vijayaraghavan, K., Montgomery, S.,Bassett, M. and Morrell, C., Biologically Erodible Microspheres asPotential Oral Drug Delivery Systems, Nature, 386:410-414, 1997. The useof polymeric microspheres for controlled drug delivery has been thesubject of a number of reviews. Langer, R., Cima, L. G., Tamada, J. A.and Wintermantel, E.: “Future Directions in Biomaterials,” Biomaterials,11:738-745, 1990.

Additional illustrative bioerodable and/or biodegradable polymersinclude polymers and copolymers of: poly(anhydride), poly(hydroxyacid)s, poly(lactone)s, poly(trimethylene carbonate), poly(glycolicacid), poly(lactic acid), poly(glycolic acid)-co-poly(glycolic acid),poly(orthocarbonate), poly(caprolactone), crosslinked biodegradablehydrogel networks like fibrin glue or fibrin sealant, caging andentrapping molecules, like cyclodextrin, molecular sieves and the like.Preferred bioerodable polymers include poly(lactic acid), poly(glycolicacid), poly(lactide), poly(glycolide), poly(lactide-co-glycolide)s,poly(caprolactone), polycarbonates, polyamides, polyanhydrides,poly(amino acid)s, poly(ortho ester)s, polyacetals, polycyanoacrylates,poly(ether ester)s, poly(dioxanone)s, poly(alkylene alkylate)s,copolymers of poly(ethylene glycol) and poly(ortho ester), degradablepolyurethanes and copolymers and blends thereof. Illustrativebioerodable polymers are further described in U.S. Patent ApplicationNos. 20020015720 and 20020034533.

In certain embodiments of the invention, the analyte sensor membrane canbe ruptured by physical (i.e., structural) or chemical changes in theanalyte sensor membrane material itself, for example, a change caused bya temperature change. For example, the analyte sensor membrane can bemade of or include a material that expands when heated. When the analytesensor membrane is secured in a fixed position and heated, the analytesensor membrane expands until it cracks or ruptures due to the increasein volume. This embodiment permits heating of the analyte sensormembrane with minimal or no heating of the analyte sensing element, afeature that is particularly important when the analyte sensing elementcontains heat-sensitive molecules, such as proteins (e.g. glucoseoxidase), which can denature upon exposure to excessive heat.

In another embodiment of the invention, the analyte sensor membranematerial can melted (i.e., undergoes a phase change) using resistiveheating. For in vivo applications, the analyte sensor membranepreferably is composed of biocompatible copolymers, such as organichydroxy acid derivatives (e.g., lactides and lactones), which can offera range of selectable melting temperatures (see PCT WO 98/26814).Particular melting temperatures, for example between about 2° C. andabout 12° C. above normal body temperature, can be selected for theanalyte sensor membranes by proper selection of starting monomer ratiosand the resulting molecular weight of the copolymer.

In certain embodiments of the invention, the analyte sensor membrane canbe thermally stimulated to enhance degradation. For example, thekinetics of analyte sensor membrane degradation can be very slow at roomtemperature and the membrane can be essentially stable. However, thekinetics of degradation are significantly increased by increasing thetemperature of the membrane material. The absolute rate of degradationcan be selected by controlling the composition of the different analytesensor membrane material that covers the analyte sensing elements. Forexample, the degradation rate of biocompatible copolymers (e.g.,lactones and lactides) can be between several hours and several years,preferably between several weeks to several months, depending on thespecific molar ratios of the primary structural units. By using an arrayof analyte sensor membranes that covers the array of analyte sensingelements, each having a different composition, complex molecular releaseprofiles can be achieved once the device reaches a critical state, forexample a state defined by its environment.

In another embodiment of the invention, all analyte sensor membraneshave constant disintegration rates (e.g., temperature independent) andthe release profile is controlled by selection of the physicaldimensions of the analyte sensor membrane material. By fixing the rateof disintegration, the time for membrane disintegration is dependent onthe thickness of the analyte sensor membrane material. For example, inan embodiment in which all analyte sensor membranes have identicalcompositions, molecular release can be controlled by varying thethickness of the membrane.

In certain embodiments of the invention, the analyte sensor membrane isformed of a material having a yield or tensile strength beyond which thematerial fails by fracture or a material that undergoes a phase change(for example, melts) with selected changes in temperature. The materialpreferably is selected from metals, such as copper, gold, silver,platinum, and zinc; glasses; ceramics; semiconductors; and brittlepolymers, such as semicrystalline polyesters. In particular, the analytesensor membrane is in the form of a thin film, e.g., a film having athickness between about 0.1 μm and 1 μm. However, because the thicknessdepends on the particular material and the mechanism of rupture (i.e.,electrochemical vs. mechanical breakdown), thicker analyte sensormembranes, e.g., having a thickness between 1 μm and 100 μm or more, maywork better for some materials, such as certain brittle material.

As noted above, the analyte sensor membrane can be made from a pluralityof layered materials. For example, the analyte sensor membraneoptionally can be coated with an overcoat material to structurallyreinforce the rupturable material layer until the overcoat material hasbeen substantially removed by dissolving, eroding, biodegrading,oxidizing, or otherwise degrading, such as upon exposure to water invivo or in vitro. Representative suitable degradable materials includesynthetic or natural biodegradable polymers.

The optimized embodiments of the invention disclosed herein can beuniversally utilized and/or applied to a wide variety of sensor methodsand designs. Consequently, the following sections describe illustrativesensor elements, configurations and methods that can incorporate theseembodiments of the invention.

B. Diagrammatic Illustration of Typical Analyte Sensor ConfigurationEmbodiments

FIG. 2 illustrates a cross-section of a typical analyte sensor elementstructure 100 of the present invention which is protectable by thesensor protection membranes disclosed herein. The sensor element isformed from a plurality of components that are typically in the form oflayers of various conductive and non-conductive constituents disposed oneach other according to a method embodiments of the invention to producea sensor structure. The components of the sensor are typicallycharacterized herein as layers because, for example, it allows for afacile characterization of the sensor structure shown in FIG. 2.Artisans will understand however, that in certain embodiments of theinvention, the sensor constituents are combined such that multipleconstituents form one or more heterogenous layers.

The embodiment shown in FIG. 2 includes a base layer 102 to support thesensor 100. The base layer 102 can be made of a material such as a metaland/or a ceramic and/or a polymeric substrate, which may beself-supporting or further supported by another material as is known inthe art. Embodiments of the invention include a conductive layer 104which is disposed on and/or combined with the base layer 102.

Typically the conductive layer 104 comprises one or more electrodes. Anoperating sensor 100 typically includes a plurality of electrodes suchas a working electrode, a counter electrode and a reference electrode.Other embodiments may also include an electrode that performs multiplefunctions, for example one that functions as both as a reference and acounter electrode. Still other embodiments may utilize a separatereference element not formed on the sensor. Typically these electrodesare electrically isolated from each other, while situated in closeproximity to one another.

As discussed in detail below, the base layer 102 and/or conductive layer104 can be generated using many known techniques and materials. Incertain embodiments of the invention, the electrical circuit of thesensor is defined by etching the disposed conductive layer 104 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 100 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating cover layer106 such as a polymer coating is optionally disposed on portions of thesensor 100. Acceptable polymer coatings for use as the insulatingprotective cover layer 106 can include, but are not limited to,non-toxic biocompatible polymers such as silicone compounds, polyimides,biocompatible solder masks, epoxy acrylate copolymers, or the like. Inthe sensor embodiments of the present invention, one or more exposedregions or apertures 108 can be made through the cover layer 106 to openthe conductive layer 104 to the external environment and to for exampleallow an analyte such as glucose to permeate the layers of the sensorand be sensed by the sensing elements. Apertures 108 can be formed by anumber of techniques, including laser ablation, tape masking, chemicalmilling or etching or photolithographic development or the like. Incertain embodiments of the invention, during manufacture, a secondaryphotoresist can also be applied to the protective layer 106 to definethe regions of the protective layer to be removed to form theaperture(s) 108. The exposed electrodes and/or contact pads can alsoundergo secondary processing (e.g. through the apertures 108), such asadditional plating processing, to prepare the surfaces and/or strengthenthe conductive regions.

In the sensor configuration shown in FIG. 2, an analyte sensing layer110 (which is preferably a sensor chemistry layer, meaning thatmaterials in this layer undergo a chemical reaction to produce a signalthat can be sensed by the conductive layer) is disposed on one or moreof the exposed electrodes of the conductive layer 104. In particular,the sensor chemistry layer 110 is an enzyme layer. Most preferably, thesensor chemistry layer 110 comprises an enzyme capable of utilizingoxygen and/or producing hydrogen peroxide, for example the enzymeglucose oxidase. Optionally the enzyme in the sensor chemistry layer iscombined with a second carrier protein such as human serum albumin,bovine serum albumin or the like. In an illustrative embodiment, anenzyme such as glucose oxidase in the sensor chemistry layer 110 reactswith glucose to produce hydrogen peroxide, a compound which thenmodulates a current at an electrode. As this modulation of currentdepends on the concentration of hydrogen peroxide, and the concentrationof hydrogen peroxide correlates to the concentration of glucose, theconcentration of glucose can be determined by monitoring this modulationin the current. In a specific embodiment of the invention, the hydrogenperoxide is oxidized at a working electrode which is an anode (alsotermed herein the anodic working electrode), with the resulting currentbeing proportional to the hydrogen peroxide concentration. Suchmodulations in the current caused by changing hydrogen peroxideconcentrations can by monitored by any one of a variety of sensordetector apparatuses such as a universal sensor amperometric biosensordetector or one of the other variety of similar devices known in the artsuch as glucose monitoring devices produced by Medtronic MiniMed.

The analyte sensing layer 110 can be applied over portions of theconductive layer or over the entire region of the conductive layer.Typically the analyte sensing layer 110 is disposed on the workingelectrode which can be the anode or the cathode. Optionally, the analytesensing layer 110 is also disposed on a counter and/or referenceelectrode. While the analyte sensing layer 110 can be up to about 1000microns (μm) in thickness, typically the analyte sensing layer isrelatively thin as compared to those found in sensors previouslydescribed in the art, and is for example, preferably less than 1, 0.5,0.25 or 0.1 microns in thickness. As discussed in detail below,particular methods for generating a thin analyte sensing layer 110include spin coating processes, dip and dry processes, low shearspraying processes, ink-jet printing processes, silk screen processesand the like. Most particularly, the thin analyte sensing layer 110 isapplied using a spin coating process.

Typically, the analyte sensing layer 110 is coated with one or moreadditional layers. Optionally, the one or more additional layersincludes a protein layer 116 disposed upon the analyte sensing layer110. Typically, the protein layer 116 comprises a protein such asalbumin or the like. Preferably, the protein layer 116 comprises humanserum albumin. In particular embodiments of the invention, an additionallayer includes an analyte modulating layer 112 that is disposed abovethe analyte sensing layer 110 to regulate analyte contact with theanalyte sensing layer 110. For example, the analyte modulating membranelayer 112 can comprise a glucose limiting membrane, which regulates theamount of glucose that contacts an enzyme such as glucose oxidase thatis present in the analyte sensing layer. Such glucose limiting membranescan be made from a wide variety of materials known to be suitable forsuch purposes, e.g., silicone compounds such as polydimethyl siloxanes,polyurethanes, polyurea cellulose acetates, Nafion, polyester sulfonicacids (e.g. Kodak AQ), hydrogels or any other suitable hydrophilicmembranes known to those skilled in the art.

In typical embodiments of the invention, an adhesion promoter layer 114is disposed between the analyte modulating layer 112 and the analytesensing layer 110 as shown in FIG. 2 in order to facilitate theircontact and/or adhesion. In a specific embodiment of the invention, anadhesion promoter layer 114 is disposed between the analyte modulatinglayer 112 and the protein layer 116 as shown in FIG. 2 in order tofacilitate their contact and/or adhesion. The adhesion promoter layer114 can be made from any one of a wide variety of materials known in theart to facilitate the bonding between such layers. In particular, theadhesion promoter layer 114 comprises a silane compound. In alternativeembodiments, protein or like molecules in the analyte sensing layer 110can be sufficiently crosslinked or otherwise prepared to allow theanalyte modulating membrane layer 112 to be disposed in direct contactwith the analyte sensing layer 110 in the absence of an adhesionpromoter layer 114.

C. Typical Analyte Sensor Constituent Embodiments

The following disclosure provides examples of typicalelements/constituents used in the analyte sensing elements of theinvention. While these elements are described as discreet units forpurposes of clarity, those of skill in the art understand that sensorcan be designed to contain elements having a combination of some or allof the material properties and/or functions of the elements/constituentsdiscussed below (e.g. an element that serves both as a supporting baseconstituent and/or a conductive constituent and/or a matrix for theanalyte sensing constituent and which further functions as an electrodein the sensor).

Base Constituent Embodiments

Sensor embodiments of the invention typically include a base constituent(see, e.g. element 102 in FIG. 2). The term “base constituent” is usedherein according to art accepted terminology and refers to theconstituent in the apparatus that typically provides a supporting matrixfor the plurality of constituents that are stacked on top of one anotherand comprise the functioning sensor. In one form, the base constituentcomprises a thin film sheet of insulative (e.g. electrically insulativeand/or water impermeable) material. This base constituent can be made ofa wide variety of materials having desirable qualities such as waterimpermeability and hermeticity. Materials include silicon, metallic,ceramic and polymeric substrates or the like.

The base constituent may be self-supporting or further supported byanother material as is known in the art. In one embodiment of the sensorconfiguration shown in FIG. 2, the base constituent 102 comprises aceramic. In an illustrative embodiment, the ceramic base comprises acomposition that is predominantly Al₂O₃ (e.g. 96%). The use of aluminaas an insulating base constituent for use with implantable devices isdisclosed in U.S. Pat. Nos. 4,940,858, 4,678,868 and 6,472,122 which areincorporated herein by reference. The base constituent embodiments ofthe invention can further include other elements known in the art, forexample hermetical vias (see, e.g. WO 03/023388). Depending upon thespecific sensor design, the base constituent can be relatively thickconstituent (e.g. thicker than 25 microns). Alternatively, one canutilize a nonconductive ceramic, such as alumina, in thin constituents,e.g., less than about 25 microns.

Conductive Constituent Embodiments

The electrochemical sensor embodiments of the invention typicallyinclude a conductive constituent disposed upon the base constituent thatincludes at least one electrode for contacting an analyte or itsbyproduct (e.g. oxygen and/or hydrogen peroxide) to be assayed (see,e.g. element 104 in FIG. 2). The term “conductive constituent” is usedherein according to art accepted terminology and refers to electricallyconductive sensor elements such as electrodes which are capable ofmeasuring and a detectable signal and conducting this to a detectionapparatus. An illustrative example of this is a conductive constituentthat can measure an increase or decrease in current in response toexposure to a stimuli such as the change in the concentration of ananalyte or its byproduct as compared to a reference electrode that doesnot experience the change in the concentration of the analyte, acoreactant (e.g. oxygen) used when the analyte interacts with acomposition (e.g. the enzyme glucose oxidase) present in analyte sensingconstituent 110 or the reaction product of this interaction (e.g.hydrogen peroxide). Illustrative examples of such elements includeelectrodes which are capable of producing a variable detectable signalsin the presence of variable concentrations of molecules such as hydrogenperoxide or oxygen. Typically, one of these electrodes in the conductiveconstituent is a working electrode, which can be made from non-corrodingmetal, conductive polymer or carbon. A carbon working electrode may bevitreous or graphitic and can be made from a solid or a paste. Ametallic working electrode may be made from platinum group metals,including palladium or gold, or a non-corroding metallically conductingoxide, such as ruthenium dioxide. Alternatively, the electrode maycomprise a silver/silver chloride electrode composition. The workingelectrode may be a wire or a thin conducting film applied to asubstrate, for example, by coating or printing. Typically, only aportion of the surface of the metallic or carbon conductor is inelectrolytic contact with the analyte-containing solution. This portionis called the working surface of the electrode. The remaining surface ofthe electrode is typically isolated from the solution by an electricallyinsulating cover constituent 106. Examples of useful materials forgenerating this protective cover constituent 106 include polymers suchas polyimides, polytetrafluoroethylene, polyhexafluoropropylene andsilicones such as polysiloxanes.

In addition to the working electrode, the analyte sensor embodiments ofthe invention typically include a reference electrode or a combinedreference and counter electrode (also termed a quasi-reference electrodeor a counter/reference electrode). If the sensor does not have acounter/reference electrode then it may include a separate counterelectrode, which may be made from the same or different materials as theworking electrode. Typical sensor embodiments of the present inventionhave one or more working electrodes and one or more counter, reference,and/or counter/reference electrodes. One embodiment of the sensor of thepresent invention has two, three or four or more working electrodes.These working electrodes in he sensor may be integrally connected orthey may be kept separate.

Typically, for in vivo use the analyte sensors of the present inventionare implanted subcutaneously in the skin of a mammal for direct contactwith the body fluids of the mammal, such as blood. Alternatively, thesensors can be implanted into other regions within the body of a mammalsuch as in the Intraperotineal space. When multiple working electrodesare used, they may be implanted together or at different positions inthe body. The counter, reference, and/or counter/reference electrodesmay also be implanted either proximate to the working electrode(s) or atother positions within the body of the mammal.

Analyte Sensing Constituent Embodiments

The electrochemical sensor embodiments of the invention include aanalyte sensing constituent disposed on the electrodes of the sensor(see, e.g. element 110 in FIG. 2). The term “analyte sensingconstituent” is used herein according to art accepted terminology andrefers to a constituent comprising a material that is capable ofrecognizing or reacting with an analyte whose presence is to be detectedby the analyte sensor apparatus. Typically, this material in the analytesensing constituent produces a detectable signal after interacting withthe analyte to be sensed, typically via the electrodes of the conductiveconstituent. In this regard the analyte sensing constituent and theelectrodes of the conductive constituent work in combination to producethe electrical signal that is read by an apparatus associated with theanalyte sensor. Typically, the analyte sensing constituent comprises anenzyme capable of reacting with and/or producing a molecule whose changein concentration can be measured by measuring the change in the currentat an electrode of the conductive constituent (e.g. oxygen and/orhydrogen peroxide), for example the enzyme glucose oxidase. An enzymecapable of producing a molecule such as hydrogen peroxide can bedisposed on the electrodes according to a number of processes known inthe art. The analyte sensing constituent can coat all or a portion ofthe various electrodes of the sensor. In this context, the analytesensing constituent may coat the electrodes to an equivalent degree.Alternatively, the analyte sensing constituent may coat differentelectrodes to different degrees, with for example the coated surface ofthe working electrode being larger than the coated surface of thecounter and/or reference electrode.

Typical sensor embodiments of this element of the invention utilize anenzyme (e.g. glucose oxidase) that has been combined with a secondprotein (e.g. albumin) in a fixed ratio (e.g. one that is typicallyoptimized for glucose oxidase stabilizing properties) and then appliedon the surface of an electrode to form a thin enzyme constituent. In atypical embodiment, the analyte sensing constituent comprises a GOx andHSA (Human Serum Albumin) mixture. In these typical embodiments, the GOxreacts with glucose present in the sensing environment (e.g. the body ofa mammal) and generates hydrogen peroxide according the reaction shownin FIG. 1, wherein the hydrogen peroxide so generated is anodicallydetected at the working electrode in the conductive constituent. Asdiscussed for example in U.S. patent application Ser. No. 10/273,767(incorporated herein by reference) extremely thin sensor chemistryconstituents are preferred and can be applied to the surface of theelectrode matrix by processes known in the art such as spin coating. Inan illustrative embodiment, glucose oxidase/albumin is prepared in aphysiological solution (e.g., phosphate buffered saline at neutral pH)with the albumin being present in an range of about 0.5%-10% by weight.Optionally the stabilized glucose oxidase constituent that is formed onthe analyte sensing constituent is very thin as compared to thosepreviously described in the art, for example less than 2, 1, 0.5, 0.25or 0.1 microns in thickness. One illustrative embodiment of theinvention utilizes a stabilized glucose oxidase constituent for coatingthe surface of an electrode wherein the glucose oxidase is mixed with acarrier protein in a fixed ratio within the constituent, and the glucoseoxidase and the carrier protein are distributed in a substantiallyuniform manner throughout the constituent. In particular, theconstituent is less than 2 microns in thickness. Surprisingly, sensorshaving these extremely thin analyte sensing constituents have materialproperties that exceed those of sensors having thicker coatingsincluding enhanced longevity, linearity, regularity as well as improvedsignal to noise ratios. While not being bound by a specific scientifictheory, it is believed that sensors having extremely thin analytesensing constituents have surprisingly enhanced characteristics ascompared to those of thicker constituents because in thicker enzymeconstituents only a fraction of the reactive enzyme within theconstituent is able to access the analyte to be sensed. In sensorsutilizing glucose oxidase, the thick coatings produced byelectrodeposition may hinder the ability of hydrogen peroxide generatedat the reactive interface of a thick enzyme constituent to contact thesensor surface and thereby generate a signal.

As noted above, the enzyme and the second protein are typically treatedto form a crosslinked matrix (e.g. by adding a cross-linking agent tothe protein mixture). As is known in the art, crosslinking conditionsmay be manipulated to modulate factors such as the retained biologicalactivity of the enzyme, its mechanical and/or operational stability.Illustrative crosslinking procedures are described in U.S. patentapplication Ser. No. 10/335,506 and PCT publication WO 03/035891 whichare incorporated herein by reference. For example, an aminecross-linking reagent, such as, but not limited to, glutaraldehyde, canbe added to the protein mixture. The addition of a cross-linking reagentto the protein mixture creates a protein paste. The concentration of thecross-linking reagent to be added may vary according to theconcentration of the protein mixture. While glutaraldehyde is apreferred crosslinking reagent, other cross-linking reagents may also beused or may be used in place of glutaraldehyde, including, but notlimited to, an amine reactive, homofunctional, cross-linking reagentsuch as Disuccinimidyl Suberate (DSS). Another example is 1-Ethyl-3(3-Dimethylaminopropyl) Carbodiimide (EDC), which is a zero-lengthcross-linker. EDC forms an amide bond between carboxylic acid and aminegroups. Other suitable cross-linkers also may be used, as will beevident to those skilled in the art.

The GOx and/or carrier protein concentration may vary for differentembodiments of the invention. For example, the GOx concentration may bewithin the range of approximately 50 mg/ml (approximately 10,000 U/ml)to approximately 700 mg/ml (approximately 150,000 U/ml). In particular,the GOx concentration is about 115 mg/ml (approximately 22,000 U/ml). Insuch embodiments, the HSA concentration may vary between about 0.5%-30%(w/v), depending on the GOx concentration. In particular, the HSAconcentration is about 1-10% w/v, and most particularly is about 5% w/v.In alternative embodiments of the invention, collagen or BSA (BovineSerum Albumin) or other structural proteins used in these contexts canbe used instead of or in addition to HSA. Although GOx is discussed asan enzyme in the analyte sensing constituent, other proteins and/orenzymes may also be used or may be used in place of GOx, including, butnot limited to glucose dehydrogenase or hexokinase, hexose oxidase,lactate oxidase, and the like. Other proteins and/or enzymes may also beused, as will be evident to those skilled in the art. Moreover, althoughHSA is employed in the example embodiment, other structural proteins,such as BSA, collagens or the like, can be used instead of or inaddition to HSA.

For embodiments employing enzymes other than GOx, concentrations otherthan those discussed herein may be utilized. For example, depending onthe enzyme employed, concentrations ranging from approximately 10%weight per weight to 70% weight per weight may be suitable. Theconcentration may be varied not only depending on the particular enzymebeing employed, but also depending on the desired properties of theresulting protein matrix. For example, a certain concentration may beutilized if the protein matrix is to be used in a diagnostic capacitywhile a different concentration may be utilized if certain structuralproperties are desired. Those skilled in the art will understand thatthe concentration utilized may be varied through experimentation todetermine which concentration (and of which enzyme or protein) may yieldthe desired result.

As noted above, in particular embodiments of the invention, the analytesensing constituent includes a composition (e.g. glucose oxidase)capable of producing a signal (e.g. a change in oxygen and/or hydrogenperoxide concentrations) that can be sensed by the electricallyconductive elements (e.g. electrodes which sense changes in oxygenand/or hydrogen peroxide concentrations). However, other useful analytesensing constituents can be formed from any composition that is capableof producing a detectable signal that can be sensed by the electricallyconductive elements after interacting with a target analyte whosepresence is to be detected. In certain embodiments, the compositioncomprises an enzyme that modulates hydrogen peroxide concentrations uponreaction with an analyte to be sensed. Alternatively, the compositioncomprises an enzyme that modulates oxygen concentrations upon reactionwith an analyte to be sensed. In this context, a wide variety of enzymesthat either use or produce hydrogen peroxide and/or oxygen in a reactionwith a physiological analyte are known in the art and these enzymes canbe readily incorporated into the analyte sensing constituentcomposition. A variety of other enzymes known in the art can produceand/or utilize compounds whose modulation can be detected byelectrically conductive elements such as the electrodes that areincorporated into the preferred sensor designs described herein. Suchenzymes include for example, enzymes specifically described in Table 1,pages 15-29 and/or Table 18, pages 111-112 of Protein Immobilization:Fundamentals and Applications (Bioprocess Technology, Vol 14) by RichardF. Taylor (Editor) Publisher: Marcel Dekker; (Jan. 7, 1991) the entirecontents of which are incorporated herein by reference.

Other useful analyte sensing constituents can be formed to includeantibodies whose interaction with a target analyte is capable ofproducing a detectable signal that can be sensed by the electricallyconductive elements after interacting with the target analyte whosepresence is to be detected. For example U.S. Pat. No. 5,427,912 (whichis incorporated herein by reference) describes an antibody-basedapparatus for electrochemically determining the concentration of ananalyte in a sample. In this device, a mixture is formed which includesthe sample to be tested, an enzyme-acceptor polypeptide, an enzyme-donorpolypeptide linked to an analyte analog (enzyme-donor polypeptideconjugate), a labeled substrate, and an antibody specific for theanalyte to be measured. The analyte and the enzyme-donor polypeptideconjugate competitively bind to the antibody. When the enzyme-donorpolypeptide conjugate is not bound to antibody, it will spontaneouslycombine with the enzyme acceptor polypeptide to form an active enzymecomplex. The active enzyme then hydrolyzes the labeled substrate,resulting in the generation of an electroactive label, which can then beoxidized at the surface of an electrode. A current resulting from theoxidation of the electroactive compound can be measured and correlatedto the concentration of the analyte in the sample. U.S. Pat. No.5,149,630 (which is incorporated herein by reference) describes anelectrochemical specific binding assay of a ligand (e.g., antigen,hapten or antibody) wherein at least one of the components isenzyme-labelled, and which includes the step of determining the extentto which the transfer of electrons between the enzyme substrate and anelectrode, associated with the substrate reaction, is perturbed bycomplex formation or by displacement of any ligand complex relative tounbound enzyme-labelled component. The electron transfer is aided byelectron-transfer mediators which can accept electrons from the enzymeand donate them to the electrode or vice versa (e.g. ferrocene) or byelectron-transfer promoters which retain the enzyme in close proximitywith the electrode without themselves taking up a formal charge. U.S.Pat. No. 5,147,781 (which is incorporated herein by reference) describesan assay for the determination of the enzyme lactate dehydrogenase-5(LDH5) and to a biosensor for such quantitative determination. The assayis based on the interaction of this enzyme with the substrate lacticacid and nicotine-amine adenine dinucleotide (NAD) to yield pyruvic acidand the reduction product of NAD. Anti-LDH5 antibody is bound to asuitable glassy carbon electrode, this is contacted with the substratecontaining LDH5, rinsed, inserted into a NAD solution, connected to anamperometric system, lactic acid is added and the current changes aremeasured, which are indicative of the quantity of LDH-5. U.S. Pat. No.6,410,251 (which is incorporated herein by reference) describes anapparatus and method for detecting or assaying one constituting memberin a specific binding pair, for example, the antigen in anantigen/antibody pair, by utilizing specific binding such as bindingbetween an antigen and an antibody, together with redox reaction fordetecting a label, wherein an oxygen micro-electrode with a sensingsurface area is used. In addition, U.S. Pat. No. 4,402,819 (which isincorporated herein by reference) describes an antibody-selectivepotentiometric electrode for the quantitative determination ofantibodies (as the analyte) in dilute liquid serum samples employing aninsoluble membrane incorporating an antigen having bonded thereto an ioncarrier effecting the permeability of preselected cations therein, whichpermeability is a function of specific antibody concentrations inanalysis, and the corresponding method of analysis. For relateddisclosures, see also U.S. Pat. Nos. 6,703,210, 5,981,203, 5,705,399 and4,894,253, the contents of which are incorporated herein by reference.

In addition to enzymes and antibodies, other exemplary materials for usein the analyte sensing constituents of the sensors disclosed hereininclude polymers that bind specific types of cells or cell components(e.g. polypeptides, carbohydrates and the like); single-strand DNA;antigens and the like. The detectable signal can be, for example, anoptically detectable change, such as a color change or a visibleaccumulation of the desired analyte (e.g., cells). Sensing elements canalso be formed from materials that are essentially non-reactive (i.e.,controls). The foregoing alternative sensor elements are beneficiallyincluded, for example, in sensors for use in cell-sorting assays andassays for the presence of pathogenic organisms, such as viruses (HIV,hepatitis-C, etc.), bacteria, protozoa and the like.

Also contemplated are analyte sensors that measure an analyte that ispresent in the external environment and that can in itself produce ameasurable change in current at an electrode. In sensors measuring suchanalytes, the analyte sensing constituent can be optional.

Protein Layer Constituent Embodiments

The electrochemical sensor embodiments of the invention optionallyinclude a protein layer constituent disposed between the analyte sensingconstituent and the analyte modulating constituent (see, e.g. element116 in FIG. 2). The term “protein layer constituent” is used hereinaccording to art accepted terminology and refers to constituentcontaining a carrier protein or the like that is selected forcompatibility with the analyte sensing constituent and or the analytemodulating constituent. In typical embodiments, the protein constituentcomprises an albumin such as human serum albumin (HSA). The HSAconcentration may vary between about 0.5%-30% (w/v). Preferably the HSAconcentration is about 1-10% w/v, and most preferably is about 5% w/v.In alternative embodiments of the invention, collagen or BSA (BovineSerum Albumin) or other structural proteins used in these contexts canbe used instead of or in addition to HSA. This constituent is typicallycrosslinked on the analyte sensing constituent according to art acceptedprotocols.

Adhesion Promoting Constituent Embodiments

The electrochemical sensor embodiments of the invention can include oneor more adhesion promoting (AP) constituents (see, e.g. element 114 inFIG. 2). The term “adhesion promoting constituent” is used hereinaccording to art accepted terminology and refers to a constituent thatincludes materials selected for their ability to promote adhesionbetween adjoining constituents in the sensor. Typically, the adhesionpromoting constituent is disposed between the analyte sensingconstituent and the analyte modulating constituent. In particular, theadhesion promoting constituent is disposed between the optional proteinconstituent and the analyte modulating constituent. The adhesionpromoter constituent can be made from any one of a wide variety ofmaterials known in the art to facilitate the bonding between suchconstituents and can be applied by any one of a wide variety of methodsknown in the art. In particular, the adhesion promoter constituentcomprises a silane compound such as γ-aminopropyltrimethoxysilane.

The use of silane coupling reagents, especially those of the formulaR′Si(OR)₃ in which R′ is typically an aliphatic group with a terminalamine and R is a lower alkyl group, to promote adhesion is known in theart (see, e.g. U.S. Pat. No. 5,212,050 which is incorporated herein byreference). For example, chemically modified electrodes in which asilane such as γ-aminopropyltriethoxysilane and glutaraldehyde were usedin a step-wise process to attach and to co-crosslink bovine serumalbumin (BSA) and glucose oxidase (GO_(x)) to the electrode surface arewell known in the art (see, e.g. Yao, T. Analytica Chim. Acta 1983, 148,27-33).

In certain embodiments of the invention, the adhesion promotingconstituent further comprises one or more compounds that can also bepresent in an adjacent constituent such as the polydimethyl siloxane(PDMS) compounds that serves to limit the diffusion of analytes such asglucose through the analyte modulating constituent. In illustrativeembodiments the formulation comprises 0.5-20% polydimethyl siloxane(PDMS), preferably 5-15% PDMS, and most preferably 10% PDMS. In otherembodiments of the invention, the adhesion promoting constituentincludes an agent selected for its ability to crosslink a siloxanemoiety present in a proximal constituent such as the analyte modulatingconstituent. In closely related embodiments of the invention, theadhesion promoting constituent includes an agent selected for itsability to crosslink an amine or carboxyl moiety of a protein present ina proximal constituent such a the analyte sensing constituent and/or theprotein constituent.

Analyte Modulating Constituent Embodiments

The electrochemical sensor embodiments of the invention include ananalyte modulating constituent disposed on the sensor (see, e.g. element112 in FIG. 2). The term “analyte modulating constituent” is used hereinaccording to art accepted terminology and refers to a constituent thattypically forms a membrane on the sensor that operates to modulate thediffusion of one or more analytes, such as glucose, through theconstituent. In certain embodiments of the invention, the analytemodulating constituent is an analyte limiting membrane which operates toprevent or restrict the diffusion of one or more analytes, such asglucose, through the constituents. In other embodiments of theinvention, the analyte modulating constituent operates to facilitate thediffusion of one or more analytes, through the constituents. Optionally,such analyte modulating constituents can be formed to prevent orrestrict the diffusion of one type of molecule through the constituent(e.g. glucose), while at the same time allowing or even facilitating thediffusion of other types of molecules through the constituent (e.g. O₂).

With respect to glucose sensors, in known enzyme electrodes, glucose andoxygen from blood, as well as some interferants, such as ascorbic acidand uric acid diffuse through a primary membrane of the sensor. As theglucose, oxygen and interferants reach the analyte sensing constituent,an enzyme, such as glucose oxidase, catalyzes the conversion of glucoseto hydrogen peroxide and gluconolactone. The hydrogen peroxide maydiffuse back through the analyte modulating constituent, or it maydiffuse to an electrode where it can be reacted to form oxygen and aproton to produce a current that is proportional to the glucoseconcentration. The sensor membrane assembly serves several functions,including selectively allowing the passage of glucose through. In thiscontext, an analyte modulating constituent is a semi-permeable membranewhich permits passage of water, oxygen and at least one selectiveanalyte and which has the ability to absorb water, the membrane having awater soluble, hydrophilic polymer.

A variety of illustrative analyte modulating compositions are known inthe art and are described for example in U.S. Pat. Nos. 6,319,540,5,882,494, 5,786,439 5,777,060, 5,771,868 and 5,391,250, the disclosuresof each being incorporated herein by reference. The hydrogels describedtherein are particularly useful with a variety of implantable devicesfor which it is advantageous to provide a surrounding water constituent.In certain embodiments of the invention, the analyte modulatingcomposition includes polydimethyl siloxane (PDMS). In other embodimentsof the invention, the analyte modulating constituent includes an agentselected for its ability to crosslink a siloxane moiety present in aproximal constituent. In closely related embodiments of the invention,the adhesion promoting constituent includes an agent selected for itsability to crosslink an amine or carboxyl moiety of a protein present ina proximal constituent.

Cover Constituent Embodiments

The electrochemical sensor embodiments of the invention include one ormore cover constituents which are typically electrically insulatingprotective constituents (see, e.g. element 106 in FIG. 2). Typically,such cover constituents are disposed on at least a portion of theanalyte modulating constituent. Acceptable polymer coatings for use asthe insulating protective cover constituent can include, but are notlimited to, non-toxic biocompatible polymers such as silicone compounds,polyimides, biocompatible solder masks, epoxy acrylate copolymers, orthe like. Further, these coatings can be photo-imageable to facilitatephotolithographic forming of apertures through to the conductiveconstituent. A typical cover constituent comprises spun on silicone. Asis known in the art, this constituent can be a commercially availableRTV (room temperature vulcanized) silicone composition. A typicalchemistry in this context is polydimethyl siloxane (acetoxy based).

Various illustrative embodiments of the invention and theircharacteristics are discussed in detail in the following sections.

D. Illustrative Embodiments of Analyte Sensor Apparatus and AssociatedCharacteristics

The analyte sensor apparatus disclosed herein has a number ofembodiments. A general embodiment of the invention is an analyte sensorapparatus for implantation within a mammal. While the analyte sensorsare typically designed to be implantable within the body of a mammal,the sensor are not limited to any particular environment can instead beused in a wide variety of contexts, for example for the analysis of mostliquid samples including biological fluids such as whole-blood, lymph,plasma, serum, saliva, urine, stool, perspiration, mucus, tears,cerebrospinal fluid, nasal secretion, cervical or vaginal secretion,semen, pleural fluid, amniotic fluid, peritoneal fluid, middle earfluid, joint fluid, gastric aspirate or the like. In addition, solid ordesiccated samples may be dissolved in an appropriate solvent to providea liquid mixture suitable for analysis.

As noted above, the sensor embodiments disclosed herein can be used tosense analytes of interest in one or more physiological environments. Incertain embodiments for example, the sensor can be in direct contactwith interstitial fluids as typically occurs with subcutaneous sensors.The sensors of the present invention may also be part of a skin surfacesystem where interstitial glucose is extracted through the skin andbrought into contact with the sensor (see, e.g. 6,155,992 and 6,706,159which are incorporated herein by reference). In other embodiments, thesensor can be in contact with blood as typically occurs for example withintravenous sensors. The sensor embodiments of the invention furtherinclude those adapted for use in a variety of contexts. In certainembodiments for example, the sensor can be designed for use in mobilecontexts, such as those employed by ambulatory users. Alternatively, thesensor can be designed for use in stationary contexts such as thoseadapted for use in clinical settings. Such sensor embodiments includefor example those used to monitor one or more analytes present in one ormore physiological environments in a hospitalized patient.

Sensor embodiments of the invention can also be incorporated in to awide variety of medical systems known in the art. Sensor embodiments ofthe invention can be used for example in a closed loop infusion systemsdesigned to control the rate that medication is infused into the body ofa user. Such a closed loop infusion system can include a sensor and anassociated meter which generates an input to a controller which in turnoperates a delivery system (e.g. one that calculates a dose to bedelivered by a medication infusion pump). In such contexts, the meterassociated with the sensor may also transmit commands to, and be used toremotely control, the delivery system. In particular, the sensor is asubcutaneous sensor in contact with interstitial fluid to monitor theglucose concentration in the body of the user, and the liquid infused bythe delivery system into the body of the user includes insulin.Illustrative systems are disclosed for example in U.S. Pat. Nos.6,558,351 and 6,551,276; PCT Application Nos. US99/21703 and US99/22993;as well as WO 2004/008956 and WO 2004/009161, all of which areincorporated herein by reference.

Certain embodiments of the invention measure peroxide and have theadvantageous characteristic of being suited for implantation in avariety of sites in the mammal including regions of subcutaneousimplantation and intravenous implantation as well as implantation into avariety of non-vascular regions. A peroxide sensor design that allowsimplantation into non-vascular regions has advantages over certainsensor apparatus designs that measure oxygen due to the problems withoxygen noise that can occur in oxygen sensors implanted intonon-vascular regions. For example in such implanted oxygen sensorapparatus designs, oxygen noise at the reference sensor can compromisethe signal to noise ratio which consequently perturbs their ability toobtain stable glucose readings in this environment. The peroxide sensorembodiments of the invention therefore overcome the difficultiesobserved with such oxygen sensors in non-vascular regions.

Certain embodiments of the invention provide advantageous long term or“permanent” sensors which are suitable for implantation in a mammal fora time period of greater than 30 days. In particular, as is known in theart (see, e.g. ISO 10993, Biological Evaluation of Medical Devices)medical devices such as the sensors described herein can be categorizedinto three groups based on implant duration: (1) “Limited” (<24 hours),(2) “Prolonged” (24 hours-30 days), and (3) “Permanent” (>30 days). Inparticular embodiments of the invention, the design of the peroxidesensor of the invention allows for a “Permanent” implantation accordingto this categorization, i.e. >30 days. In related embodiments of theinvention, the highly stable design of the peroxide sensor of theinvention allows for an implanted sensor to continue to function in thisregard for 2, 3, 4, 5, 6 or 12 or more months.

The invention disclosed herein has a number of embodiments. A typicalembodiment of the invention is a long term sensor for measuring at leastone analyte in the body of a user, the sensor including: a housing; asensor coupled to the housing; at least one structure operativelycoupled to the sensor for relaying information away from the sensor,where the sensor includes at least one sensor array having two or moreelements that is controllable in a manner such that sensor elements maybe activated at different times to extend the useful life of the sensor.In certain embodiments of the invention, such long term analyte sensingdevices are prolonged analyte sensors. Alternatively, the analytesensing devices are permanent analyte sensors.

Another illustrative embodiment of the invention is an analyte sensingdevice for sensing at least one analyte, the analyte sensing deviceincluding: a plurality of analyte sensor elements adapted to contact andsense analyte; at least one analyte sensor membrane disposed upon atleast one of the plurality of analyte sensor elements in a manner thatreversibly prevents an analyte from contacting the at least one of theplurality of analyte sensor elements, where the permeability of theanalyte sensor membrane can be controlled to allow an analyte to contactat least one of the plurality of analyte sensor elements; and at leastone structure operatively coupled to the analyte sensing device forrelaying information away from the analyte sensing device. Optionally,the plurality of analyte sensor elements that contact and sense theanalyte are disposed in an array in the analyte sensing device.

Individual sensor elements within the plurality of the analyte sensorelements in the analyte sensing device can sense the same or differentanalytes. In this context, embodiments of the invention are adapted tomeasure multiple analytes simultaneously. For example, embodiments ofthe inventions can be adapted so that multiple individual sensorelements (e.g. those within each cavity) adapted to sense differentanalytes can be exposed to the external environment at the same time.Alternatively, multiple individual sensor elements adapted to sensedifferent analytes can be exposed to the external environment atdifferent times. Similar embodiments include an analyte sensing deviceadapted to function as multi-analyte sensor on a single chip (or,alternatively, on multiple chips). In certain contexts, a signal from anindividual analyte sensor element within the plurality of analyte sensorelements that contact and sense an analyte in the analyte sensing deviceare individually interrogated and/or read. Alternatively, multipleanalyte sensor elements within the plurality of analyte sensor elementsthat contact and sense an analyte in the analyte sensing device areinterrogated and/or read simultaneously and/or in combination.

Embodiments of the analyte sensing device include those adapted toinclude both analyte sensing elements covered by a analyte sensormembrane and, in addition, include one or more reservoirs that are alsocovered by a controllable analyte sensor membrane and which contain oneor more compounds that can be controllably released from the reservoirto, for example, facilitate the activity of the analyte sensing device.In one such embodiment, such reservoirs can include solutions thatfunction as calibration fluids (e.g. fluids having defined analyteconcentrations) for an analyte sensing element within the analytesensing device. Examples of such calibration fluids include fluidscontaining define glucose and/or lactate concentrations (i.e. forglucose and/or lactate sensors). In such embodiments of the invention, acalibration fluid from one or more reservoirs can be released in amanner that exposes them to the analyte sensing element(s) in the deviceand in this way calibrate each of the analyte sensors. In certainembodiments of the invention, cavities containing such fluids can beco-localized with the sensing elements.

Embodiments of the invention that are adapted to include barriermembranes that reversibly cover both analyte sensing elements as wellreservoirs containing compounds that can be controllably released intothe environment, include those where the compounds in the reservoirs aredesigned to enhance the function of the analyte sensing device by, forexample, reshaping and/or adapting the in vivo tissue environment intowhich the sensing device is placed. In one such embodiment of theinvention, the reservoir can contain a compound that is designed todecrease the host response that can occur with the implantation ofmedical devices. Such compounds can include any one of a wide variety ofsuch compounds known in the art, for example hormones that decreasecellular responses and/or antibiotics such as rapamycin. Such compoundsinclude “growth inhibitory agents” which are compounds or compositionswhich inhibit growth of a cell in vitro and/or in vivo. Thus,illustrative growth inhibitory agent may be those which significantlyreduce the percentage of cells in S phase. Examples of growth inhibitoryagents include agents that block cell cycle progression (at a placeother than S phase), such as agents that induce G1 arrest and M-phasearrest. Classical M-phase blockers include the vincas (vincristine andvinblastine), TAXOL®, and topo II inhibitors such as doxorubicin,epirubicin, daunorubicin, etoposide, and bleomycin. Those agents thatarrest G1 also spill over into S-phase arrest, for example, DNAalkylating agents such as tamoxifen, prednisone, dacarbazine,mechlorethamine, cisplatin, methotrexate, 5-fluorouracil, and ara-C

In alternative embodiments of the invention, the reservoir can contain acompound designed effect the implantation site, for example to enhance“vascularity” at a tissue site in a manner that enhances the transportof analyte to the analyte sensing element. Such compounds can includeany one of a wide variety of such compounds known in the art, forexample cytokines. In this context, “cytokine” means those proteinsreleased by one cell population which act on another cell asintercellular mediators. Examples of such cytokines are lymphokines,monokines, and traditional polypeptide hormones. Included among thecytokines are growth hormone such as human growth hormone, N-methionylhuman growth hormone, and bovine growth hormone; parathyroid hormone;thyroxine; insulin; proinsulin; relaxin; prorelaxin; glycoproteinhormones such as follicle stimulating hormone (FSH), thyroid stimulatinghormone (TSH), and luteinizing hormone (LH); hepatic growth factor;fibroblast growth factor; prolactin; placental lactogen; tumor necrosisfactor-alpha and -beta; mullerian-inhibiting substance; mousegonadotropin-associated peptide; inhibin; activin; vascular endothelialgrowth factor; integrin; thrombopoietin (TPO); nerve growth factors suchas NGF-alpha; platelet-growth factor; transforming growth factors (TGFs)such as TGF-alpha and TGF-beta; insulin-like growth factor-I and -II;erythropoietin (EPO); osteoinductive factors; interferons such asinterferon-alpha, -beta and -gamma colony stimulating factors (CSFs)such as macrophage-CSF (M-CSF); granulocyte-macrophage-CSF (GM-CSF); andgranulocyte-CSF (G-CSF); interleukins (ILs) such as IL-1, IL-1 alpha,IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-11, IL-12; atumor necrosis factor such as TNF-alpha or TNF-beta; and otherpolypeptide factors including LIF and kit ligand (KL). As used herein,the term cytokine includes proteins from natural sources or fromrecombinant cell culture and biologically active equivalents of thenative sequence cytokines.

Other embodiments of the invention include those where the reservoir cancontain a series of compounds that are released at different times in amanner that enhances drug delivery an/or the tissue environmentsurrounding a device to create the most optimal environment. Assubcutaneous and peritoneal tissue are well known for aggressive hostresponse—certain embodiments of the invention include compounds thatdiminish the host response in the immediate period post implantation(e.g. the first few hours) through some drug; and then subsequentlypromote blood vessel formation near the implant during the extendedperiod post implantation (e.g. the next few weeks), while at the sametime minimizing a potential fibrous capsule formation. In suchembodiments, various reservoirs in the analyte sensing device cancontain a series of different compounds that are released according to apredetermined drug profile.

In certain embodiments of the invention, the analyte sensing element inthe analyte sensor device/apparatus includes, but is not limited to, abase layer and a conductive layer disposed upon the base layer where theconductive layer includes a working electrode and preferably a referenceelectrode and a counter electrode. In this embodiment of the invention,an analyte sensing layer is disposed on the conductive layer. Typically,the analyte sensing layer comprises a composition that detectably altersthe electrical current at the working electrode in the conductive layerin the presence of an analyte. Illustrative examples of suchcompositions include enzymes such as glucose oxidase, glucosedehydrogenase, lactate oxidase, hexokinase and lactose dehydrogenase orthe like (e.g. any other protein and/or polymer and/or a combinationthereof that stabilizes the enzyme layer). This embodiment of theinvention optionally includes a protein layer disposed on the analytesensing layer, with this protein layer typically including a carrierprotein such as bovine serum albumin or human serum albumin or the like.In this embodiment, an adhesion promoting layer is disposed on theanalyte sensing layer or the optional protein layer, which serves topromotes the adhesion between the analyte sensing layer and one or moreproximal sensor layers. In particular, this adhesion promoting layerincludes a silane composition selected for its ability to enhance thestability of the sensor structure, for exampleγ-aminopropyltrimethoxysilane. This embodiment also includes an analytemodulating layer disposed above the analyte sensing layer, where theanalyte modulating layer modulates the diffusion of the analyte through,for example a glucose limiting membrane. This embodiment also includes ainsulative cover layer disposed on at least a portion of the analytemodulating layer, where the cover layer further includes an aperturethat exposes at least a portion of the analyte modulating layer to asolution comprising the analyte to be sensed. In particular, the analytesensor apparatus is designed to function via anodic polarization suchthat the alteration in current can be detected at the working electrode(anode) in the conductive layer of the analyte sensor apparatus; and thealteration in current that can be detected at this working anode can becorrelated with the concentration of the analyte.

In the device embodiments of the invention, the permeability of theanalyte sensor membrane is typically controlled so that a second analytesensor element in the plurality of analyte sensor elements contactsanalyte after a first analyte sensor element in the plurality of analytesensor elements exhibits a decrease in the ability to sense analyte dueto biofouling and/or loss of activity of an analyte sensing enzymedisposed in the first analyte sensor element, so that the useful life ofthe analyte sensing device is extended. In certain embodiments of theinvention, the analyte sensing device is implantable within the body ofa mammal. Optionally, the analyte sensed is glucose and/or lactate.

As discussed in detail below, the analyte sensor membrane can be madeusing a number of different methods and materials know in the art. Forexample, in one embodiment, the analyte sensor membrane comprises arupturable metallic membrane that hermetically seals the analyte sensorelement. Alternatively, the analyte sensor membrane comprises abiodegradable polymer that degrades at a defined rate within an in vivoenvironment.

In certain embodiments of the invention, the analyte sensor membranesand/or the analyte sensing elements are discreetly controlled to allowrupture of a specific membrane and/or interrogation and receipt ofsignal from a specific analyte sensing element. In certain embodimentsof the invention, the plurality of analyte sensor elements has aplurality of different analyte sensor membranes disposed thereon.Alternatively, the plurality of analyte sensor elements have similar oridentical analyte sensor membranes disposed thereon. Optionally, atleast one of the analyte sensor elements in the analyte sensing devicecomprises a hydrogel disposed thereon, wherein upon exposure to anaqueous solution, the hydrogel expands in a manner that increases thepermeability of the analyte sensor membrane.

Another embodiment of the invention is a method of making a sensorapparatus for implantation within a mammal including the steps of:providing a plurality of analyte sensor elements adapted to contact andsense analyte; providing at least one analyte sensor membrane disposedupon at least one of the plurality of analyte sensor elements in amanner that reversibly prevents an analyte from contacting the at leastone of the plurality of analyte sensor elements, where the permeabilityof the analyte sensor membrane can be controlled to allow an analyte tocontact at least one of the plurality of analyte sensor elements; andproviding at least one structure operatively coupled to the analytesensing device for relaying information away from the analyte sensingdevice.

Yet another embodiment of the invention is a method of extending theuseful life of an analyte sensing device including analyte sensorelements that exhibit a decrease in the ability to sense analyte overtime due to biofouling or a loss of activity of an analyte sensingenzyme disposed on an analyte sensor element; the method includingsensing an analyte with an analyte sensing device including: a pluralityof analyte sensor elements adapted to contact and sense analyte; atleast one analyte sensor membrane disposed upon at least one of theplurality of analyte sensor elements in a manner that reversiblyprevents an analyte from contacting the at least one of the plurality ofanalyte sensor elements, where the permeability of the analyte sensormembrane can be controlled to allow an analyte to contact at least oneof the plurality of analyte sensor elements; and at least one structureoperatively coupled to the analyte sensing device for relayinginformation away from the analyte sensing device; where the useful lifeof an analyte sensing device is extended by: deactivating a firstanalyte sensor element in the plurality of analyte sensor elements thatcontact and sense analyte when the first analyte sensing elementexhibits a decrease in the ability to sense analyte due to biofouling ora loss of activity of an analyte sensing enzyme disposed on the firstanalyte sensor element; and activating a second analyte sensor elementin the plurality of analyte sensor elements adapted to contact and senseanalyte by controlling the permeability of an analyte sensor membranedisposed upon the second analyte sensor element to allow an analyte tocontact the second analyte sensor element, so that the useful life ofthe analyte sensing device is extended.

The various components of the analyte sensing devices disclosed hereincan be arranged in a variety of configurations. For example in certainembodiments of the invention, at least one of the plurality of analytesensor elements is disposed in a reservoir or well. Alternatively, atleast one of the plurality of analyte sensor elements is not disposed ina reservoir or well. This second configuration is favored for example insituations where a reservoir or well structure in an implantable analytesensing device acts as a trap for debris (e.g. cellular components etc.)that accelerates biofouling of an analyte sensor elements. Consequently,by eliminating the reservoir or well, for example by having the analytesensing element flush with a housing in which it is placed, biofoulingof the analyte sensing elements is inhibited.

E. Permutations of Analyte Sensor Apparatus and Element Embodiments

As noted above, the invention disclosed herein encompasses a variety ofsensor embodiments, all of which can be covered by one or more sensorprotection membranes. Such embodiments of the invention allow artisansto generate a variety of permutations of the analyte sensor apparatusdisclosed herein. As noted above, illustrative general embodiments ofthe sensor disclosed herein include a base layer, a cover layer and atleast one layer having a sensor element such as an electrode disposedbetween the base and cover layers. Typically, an exposed portion of oneor more sensor elements (e.g., a working electrode, a counter electrode,reference electrode, etc.) is coated with a very thin layer of materialhaving an appropriate electrode chemistry. For example, an enzyme suchas lactate oxidase, glucose oxidase, glucose dehydrogenase orhexokinase, can be disposed on the exposed portion of the sensor elementwithin an opening or aperture defined in the cover layer. FIG. 2illustrates a cross-section of a typical sensor structure 100 of thepresent invention. The sensor is formed from a plurality of layers ofvarious conductive and non-conductive constituents disposed on eachother according to a method of the invention to produce a sensorstructure 100.

As noted above, in the sensors of the invention, the various layers(e.g. the analyte sensing layer) of the sensors can have one or morebioactive and/or inert materials incorporated therein. The term“incorporated” as used herein is meant to describe any state orcondition by which the material incorporated is held on the outersurface of or within a solid phase or supporting matrix of the layer.Thus, the material “incorporated” may, for example, be immobilized,physically entrapped, attached covalently to functional groups of thematrix layer(s). Furthermore, any process, reagents, additives, ormolecular linker agents which promote the “incorporation” of saidmaterial may be employed if these additional steps or agents are notdetrimental to, but are consistent with the objectives of the presentinvention. This definition applies, of course, to any of the embodimentsof the present invention in which a bioactive molecule (e.g. an enzymesuch as glucose oxidase) is “incorporated.” For example, Certain layersof the sensors disclosed herein include a proteinaceous substance suchas albumin which serves as a crosslinkable matrix. As used herein, aproteinaceous substance is meant to encompass substances which aregenerally derived from proteins whether the actual substance is a nativeprotein, an inactivated protein, a denatured protein, a hydrolyzedspecies, or a derivatized product thereof. Examples of suitableproteinaceous materials include, but are not limited to enzymes such asglucose oxidase and lactate oxidase and the like, albumins (e.g. humanserum albumin, bovine serum albumin etc.), caseins, gamma-globulins,collagens and collagen derived products (e.g., fish gelatin, fish glue,animal gelatin, and animal glue).

A particular embodiment of the invention is shown in FIG. 2. Thisembodiment includes an electrically insulating base layer 102 to supportthe sensor 100. The electrically insulating layer base 102 can be madeof a material such as a ceramic substrate, which may be self-supportingor further supported by another material as is known in the art. In analternative embodiment, the electrically insulating layer 102 comprisesa polyimide substrate, for example a polyimide tape, dispensed from areel. Providing the layer 102 in this form can facilitate clean, highdensity mass production. Further, in some production processes usingsuch a polyimide tape, sensors 100 can be produced on both sides of thetape.

Typical embodiments of the invention include an analyte sensing layerdisposed on the base layer 102. In a certain embodiment as shown in FIG.2 the analyte sensing layer comprises a conductive layer 104 which isdisposed on insulating base layer 102. In particular, the conductivelayer 104 comprises one or more electrodes. The conductive layer 104 canbe applied using many known techniques and materials as will bedescribed hereafter, however, the electrical circuit of the sensor 100is typically defined by etching the disposed conductive layer 104 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 100 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating protectivecover layer 106 such as a polymer coating is typically disposed onportions of the conductive layer 104. Acceptable polymer coatings foruse as the insulating protective layer 106 can include, but are notlimited to, non-toxic biocompatible polymers such as polyimide,biocompatible solder masks, epoxy acrylate copolymers, or the like.Further, these coatings can be photo-imageable to facilitatephotolithographic forming of apertures 108 through to the conductivelayer 104. In certain embodiments of the invention, an analyte sensinglayer is disposed upon a metallic and/or ceramic and/or polymeric matrixwith this combination of elements functioning as an electrode in thesensor.

In the sensor embodiments of the present invention, one or more exposedregions or apertures 108 can be made through the protective layer 106 tothe conductive layer 104 to define the contact pads and electrodes ofthe sensor 100. In addition to photolithographic development, theapertures 108 can be formed by a number of techniques, including laserablation, chemical milling or etching or the like. A secondaryphotoresist can also be applied to the cover layer 106 to define theregions of the protective layer to be removed to form the apertures 108.An operating sensor 100 typically includes a plurality of electrodessuch as a working electrode and a counter electrode electricallyisolated from each other, however typically situated in close proximityto one another. Other embodiments may also include a referenceelectrode. Still other embodiments may utilize an separate referenceelement not formed on the sensor. The exposed electrodes and/or contactpads can also undergo secondary processing through the apertures 108,such as additional plating processing, to prepare the surfaces and/orstrengthen the conductive regions.

A analyte sensing layer 110 is typically disposed on one or more of theexposed electrodes of the conductive layer 104 through the apertures108. In particular, the analyte sensing layer 110 is a sensor chemistrylayer and most preferably an enzyme layer. Particularly, the analytesensing layer 110 comprises the enzyme glucose oxidase or the enzymelactate oxidase. In such embodiments, the analyte sensing layer 110reacts with glucose to produce hydrogen peroxide which modulates acurrent to the electrode which can be monitored to measure an amount ofglucose present. The sensor chemistry layer 110 can be applied overportions of the conductive layer or over the entire region of theconductive layer. In particular, the sensor chemistry layer 110 isdisposed on portions of a working electrode and a counter electrode thatcomprise a conductive layer. Particular methods for generating the thinsensor chemistry layer 110 include spin coating processes, dip and dryprocesses, low shear spraying processes, ink-jet printing processes,silk screen processes and the like. Most preferably the thin sensorchemistry layer 110 is applied using a spin coating process.

The analyte sensing layer 110 is typically coated with one or morecoating layers. In particular embodiments of the invention, one suchcoating layer includes a membrane which can regulate the amount ofanalyte that can contact an enzyme of the analyte sensing layer. Forexample, a coating layer can comprise an analyte modulating membranelayer such as a glucose limiting membrane which regulates the amount ofglucose that contacts the glucose oxidase enzyme layer on an electrode.Such glucose limiting membranes can be made from a wide variety ofmaterials known to be suitable for such purposes, e.g., silicone,polyurethane, polyurea cellulose acetate, Nafion, polyester sulfonicacid (Kodak AQ), hydrogels or any other membrane known to those skilledin the art.

In particular embodiments of the invention, a coating layer is a glucoselimiting membrane layer 112 which is disposed above the sensor chemistrylayer 110 to regulate glucose contact with the sensor chemistry layer110. In some embodiments of the invention, an adhesion promoter layer114 is disposed between the membrane layer 112 and the sensor chemistrylayer 110 as shown in FIG. 2 in order to facilitate their contact and/oradhesion. The adhesion promoter layer 114 can be made from any one of awide variety of materials known in the art to facilitate the bondingbetween such layers. Preferably, the adhesion promoter layer 114comprises a silane compound. In alternative embodiments, protein or likemolecules in the sensor chemistry layer 110 can be sufficientlycrosslinked or otherwise prepared to allow the membrane layer 112 to bedisposed in direct contact with the sensor chemistry layer 110 in theabsence of an adhesion promoter layer 114.

As noted above, embodiments of the present invention can include one ormore functional coating layers. As used herein, the term “functionalcoating layer” denotes a layer that coats at least a portion of at leastone surface of a sensor, more preferably substantially all of a surfaceof the sensor, and that is capable of interacting with one or moreanalytes, such as chemical compounds, cells and fragments thereof, etc.,in the environment in which the sensor is disposed. Non-limitingexamples of functional coating layers include sensor chemistry layers(e.g., enzyme layers), analyte limiting layers, biocompatible layers;layers that increase the slipperiness of the sensor; layers that promotecellular attachment to the sensor; layers that reduce cellularattachment to the sensor; and the like. Typically, analyte modulatinglayers operate to prevent or restrict the diffusion of one or moreanalytes, such as glucose, through the layers. Optionally such layerscan be formed to prevent or restrict the diffusion of one type ofmolecule through the layer (e.g. glucose), while at the same timeallowing or even facilitating the diffusion of other types of moleculesthrough the layer (e.g. O₂). An illustrative functional coating layer isa hydrogel such as those disclosed in U.S. Pat. Nos. 5,786,439 and5,391,250, the disclosures of each being incorporated herein byreference. The hydrogels described therein are particularly useful witha variety of implantable devices for which it is advantageous to providea surrounding water layer.

The sensor embodiments disclosed herein can include layers havingUV-absorbing polymers. In accordance with one aspect of the presentinvention, there is provided a sensor including at least one functionalcoating layer including a UV-absorbing polymer. In particularembodiments, the UV-absorbing polymer is a polyurethane, a polyurea or apolyurethane/polyurea copolymer. More preferably, the selectedUV-absorbing polymer is formed from a reaction mixture including adiisocyanate, at least one diol, diamine or mixture thereof, and apolyfunctional UV-absorbing monomer.

UV-absorbing polymers are used with advantage in a variety of sensorfabrication methods, such as those described in U.S. Pat. No. 5,390,671,to Lord et al., entitled “Transcutaneous Sensor Insertion Set”; No.5,165,407, to Wilson et al., entitled “Implantable Glucose Sensor”; andU.S. Pat. No. 4,890,620, to Gough, entitled “Two-Dimensional DiffusionGlucose Substrate Sensing Electrode”, which are incorporated herein intheir entireties by reference. However, any sensor production methodwhich includes the step of forming a UV-absorbing polymer layer above orbelow a sensor element is considered to be within the scope of thepresent invention. In particular, the inventive method embodiments arenot limited to thin-film fabrication methods, and can work with othersensor fabrication methods that utilize UV-laser cutting. Embodimentscan work with thick-film, planar or cylindrical sensors and the like,and other sensor shapes requiring laser cutting.

As disclosed herein, the sensor embodiments of the present invention areparticularly designed for use as subcutaneous or transcutaneous glucosesensors for monitoring blood glucose levels in a diabetic patient.Typically, each sensor comprises a plurality of sensor elements, forexample electrically conductive elements such as elongated thin filmconductors, formed between an underlying insulative thin film base layerand an overlying insulative thin film cover layer.

If desired, a plurality of different sensor elements can be included ina single sensor. For example, both conductive and reactive sensorelements can be combined in one sensor, optionally with each sensorelement being disposed on a different portion of the base layer. One ormore control elements can also be provided. In such embodiments, thesensor can have defined in its cover layer a plurality of openings orapertures. One or more openings can also be defined in the cover layerdirectly over a portion of the base layer, in order to provide forinteraction of the base layer with one or more analytes in theenvironment in which the sensor is disposed. The base and cover layerscan be comprised of a variety of materials, typically polymers. In morespecific embodiments the base and cover layers are comprised of aninsulative material such as a polyimide. Openings are typically formedin the cover layer to expose distal end electrodes and proximal endcontact pads. In a glucose monitoring application, for example, thesensor can be placed transcutaneously so that the distal end electrodesare in contact with patient blood or extracellular fluid, and thecontact pads are disposed externally for convenient connection to amonitoring device.

The sensor embodiments of the invention can have any desiredconfiguration, for example planar or cylindrical. The base layer 102 canbe self-supportive, such as a rigid polymeric layer, or non-selfsupportive, such as a flexible film. The latter embodiment is desirablein that it permits continuous manufacture of sensors using, for example,a roll of a polymeric film which is continuously unwound and upon whichsensor elements and coating layers are continuously applied.

F. Analyte Sensor Apparatus Configuration Embodiments

In a clinical setting, accurate and relatively fast determinations ofanalytes such as glucose and/or lactate levels can be determined fromblood samples utilizing electrochemical sensors. Conventional sensorsare fabricated to be large, comprising many serviceable parts, or small,planar-type sensors which may be more convenient in many circumstances.The term “planar” as used herein refers to the well-known procedure offabricating a substantially planar structure comprising layers ofrelatively thin materials, for example, using the well-known thick orthin-film techniques. See, for example, Liu et al., U.S. Pat. No.4,571,292, and Papadakis et al., U.S. Pat. No. 4,536,274, both of whichare incorporated herein by reference. As noted below, embodiments of theinvention disclosed herein have a wider range of geometricalconfigurations (e.g. planar) than existing sensors in the art. Inaddition, certain embodiments of the invention include one or more ofthe sensors disclosed herein coupled to another apparatus such as amedication infusion pump.

An exemplary multiple sensor device comprises a single device having afirst sensor which is polarized cathodically and designed to measure thechanges in oxygen concentration that occur at the working electrode (acathode) as a result of glucose interacting with glucose oxidase; and asecond sensor which is polarized anodically and designed to measurechanges in hydrogen peroxide concentration that occurs at the workingelectrode (an anode) as a result of glucose coming form the externalenvironment and interacting with glucose oxidase. As is known in theart, in such designs, the first oxygen sensor will typically experiencea decrease in current at the working electrode as oxygen contacts thesensor while the second hydrogen peroxide sensor will typicallyexperience an increase in current at the working electrode as thehydrogen peroxide generated as shown in FIG. 1 contacts the sensor. Inaddition, as is known in the art, an observation of the change incurrent that occurs at the working electrodes as compared to thereference electrodes in the respective sensor systems correlates to thechange in concentration of the oxygen and hydrogen peroxide moleculeswhich can then be correlated to the concentration of the glucose in theexternal environment (e.g. the body of the mammal).

II. Illustrative Methods and Materials for Making Analyte SensorApparatus of the Invention

A number of articles, U.S. patents and patent application describe thestate of the art with the common methods and materials disclosed hereinand further describe various elements (and methods for theirmanufacture) that can be used in the sensor designs disclosed herein.These include for example, U.S. Pat. Nos. 6,413,393; 6,368,274;5,786,439; 5,777,060; 5,391,250; 5,390,671; 5,165,407, 4,890,620,5,390,671, 5,390,691, 5,391,250, 5,482,473, 5,299,571, 5,568,806; UnitedStates Patent Application 20020090738; as well as PCT InternationalPublication Numbers WO 01/58348, WO 03/034902, WO 03/035117, WO03/035891, WO 03/023388, WO 03/022128, WO 03/022352, WO 03/023708, WO03/036255, WO03/036310 and WO 03/074107, the contents of each of whichare incorporated herein by reference.

Typical sensors for monitoring glucose concentration of diabetics arefurther described in Shichiri, et al.: “In Vivo Characteristics ofNeedle-Type Glucose Sensor-Measurements of Subcutaneous GlucoseConcentrations in Human Volunteers,” Horm. Metab. Res., Suppl. Ser.20:17-20 (1988); Bruckel, et al.: “In Vivo Measurement of SubcutaneousGlucose Concentrations with an Enzymatic Glucose Sensor and a WickMethod,” Klin. Wochenschr. 67:491-495 (1989); and Pickup, et al.: “InVivo Molecular Sensing in Diabetes Mellitus: An Implantable GlucoseSensor with Direct Electron Transfer,” Diabetologia 32:213-217 (1989).Other sensors are described in, for example Reach, et al., in ADVANCESIN IMPLANTABLE DEVICES, A. Turner (ed.), JAI Press, London, Chap. 1,(1993), incorporated herein by reference.

III. Methods for Using Analyte Sensor Apparatus Embodiments of theInvention

One embodiment of the invention is a method of sensing an analyte withinthe body of a mammal, the method including implanting an analyte sensorin to the mammal, the analyte sensor comprising: a plurality of analytesensor elements that contact and sense analyte; at least one analytesensor membrane disposed upon at least one of the plurality of analytesensor elements in a manner that reversibly prevents an analyte fromcontacting the at least one of the plurality of analyte sensor elements,where the permeability of the analyte sensor membrane can be controlledto allow an analyte to contact at least one of the plurality of analytesensor elements; and at least one structure operatively coupled to theanalyte sensing device for relaying information away from the analytesensing device; and sensing an analyte within the body of a mammal.

A related embodiment of the invention is a method of sensing an analytewithin the body of a mammal, the method including implanting an analytesensor embodiment disclosed herein in to the mammal and then sensing analteration in current at the working electrode and correlating thealteration in current with the presence of the analyte, so that theanalyte is sensed. Typically, the analyte sensor is polarized anodicallysuch that the working electrode where the alteration in current issensed is an anode. In one such method, the analyte sensor apparatussenses glucose in the mammal. In an alternative method, the analytesensor apparatus senses a protein, lactose, a carbohydrate, asaccharide, a mineral, and element, a small molecule compound, a virus,a peptide, a protein fragment, a medication, a drug, an element of abody chemistry assay, body constituent or byproduct lactate, potassium,calcium, oxygen, pH, and/or any physiologically relevant analyte in themammal.

Certain analyte sensors having the structure discussed above have anumber of highly desirable characteristics which allow for a variety ofmethods for sensing analytes in a mammal. For example in such methods,the analyte sensor apparatus implanted in the mammal functions to sensean analyte within the body of a mammal for more than 1, 2, 3, 4, 5, or 6months. In particular, the analyte sensor apparatus so implanted in themammal senses an alteration in current in response to an analyte within15, 10, 5 or 2 minutes of the analyte contacting the sensor. In suchmethods, the sensors can be implanted into a variety of locations withinthe body of the mammal, for example in both vascular and non-vascularspaces.

IV. Kits and Sensor Set Embodiments of the Invention

In another embodiment of the invention, a kit and/or sensor set, usefulfor the sensing an analyte as is described above, is provided. The kitand/or sensor set typically includes a container, a label and an analytesensor as described above. Suitable containers include, for example, aneasy to open package made from a material such as a metal foil, bottles,vials, syringes, and test tubes. The containers may be formed from avariety of materials such as metals (e.g. foils) paper products, glassor plastic. The label on, or associated with, the container indicatesthat the sensor is used for assaying the analyte of choice. Inparticular embodiments, the container holds a plurality of analytesensing elements, one or more of which is covered by an analyte sensormembrane. The kit and/or sensor set may further include other materialsdesirable from a commercial and user standpoint, including elements ordevices designed to facilitate the introduction of the sensor into theanalyte environment, other buffers, diluents, filters, needles,syringes, and package inserts with instructions for use.

Various citations are referenced throughout the specification. Inaddition, certain text from related art is reproduced herein to moreclearly delineate the various embodiments of the invention. Thedisclosures of all citations in the specification are expresslyincorporated herein by reference.

The invention claimed is:
 1. A method of sensing an analyte within thebody of a mammal, the method comprising implanting an analyte sensor into the mammal, the analyte sensor comprising: a plurality of analytesensor elements adapted to contact and sense analyte, the analyte sensorelements comprising: a base layer; at least three working electrodesdisposed on the base layer; a glucose oxidase layer disposed upon the atleast three working electrodes, wherein the least three workingelectrodes are coated with the glucose oxidase layer so as to allow theanalyte sensing device to sense glucose; an analyte modulating layerdisposed on the glucose oxidase layer, wherein the analyte modulatinglayer comprises a hydrogel composition that includes a compound selectedfor its ability to crosslink to a siloxane composition; an adhesionpromoting layer disposed between the glucose oxidase layer and theanalyte modulating layer that functions to promote adhesion between theglucose oxidase layer and the analyte modulating layer, wherein theadhesion promoting layer comprises a siloxane composition crosslinked tothe compound in the analyte modulating layer and; at least one analytesensor membrane disposed upon at least one sealable well in a mannerthat reversibly prevents an analyte from contacting the at least one ofthe plurality of analyte sensors, wherein the permeability of theanalyte sensor membrane can be controlled to allow an analyte to contactat least one of the plurality of analyte sensor elements; and at leastone structure operatively coupled to the analyte sensing device forrelaying information away from the analyte sensing device; and sensingan analyte within the body of a mammal.
 2. The method of claim 1,further comprising controlling the analyte sensor membrane so that asecond analyte sensor element in the plurality of analyte sensorelements contacts analyte after a first analyte sensor element in theplurality of analyte sensor elements exhibits a decrease in the abilityto sense analyte due to biofouling, so that the useful life of theanalyte sensing device is extended.
 3. The method of claim 1, furthercomprising controlling the analyte sensor membrane so that a secondanalyte sensor element in the plurality of analyte sensor elements isadapted to contact analyte after a first analyte sensor element in theplurality of analyte sensor elements exhibits a decrease in the abilityto sense analyte due to loss of activity of an analyte sensing enzymedisposed in the first analyte sensor element, so that the useful life ofthe analyte sensing device is extended.
 4. The method of claim 1,wherein the analyte sensor membrane comprises a rupturable metallicmembrane.
 5. The method of claim 4, wherein the analyte sensor membranehermetically seals the analyte sensor element.
 6. The method of claim 1,wherein the analyte sensor membrane comprises a biodegradable polymer.7. The method of claim 6, wherein the biodegradable polymer is comprisedof materials selected for their ability to degrade at a defined ratewithin an in vivo environment.
 8. The method of claim 1, wherein theplurality of analyte sensor elements has a plurality of differentanalyte sensor membranes disposed thereon.
 9. The method of claim 1,further comprising controlling each of plurality of analyte sensormembranes disposed on the analyte sensor elements separately.
 10. Themethod of claim 1, wherein the analyte sensing device is a prolongedanalyte sensor.
 11. The method of claim 1, wherein the analyte sensingdevice is a permanent analyte sensor.
 12. The method of claim 1, whereinat least two of the analyte sensor elements in the analyte sensingdevice sense the same analyte.
 13. The method of claim 1, wherein atleast two of the analyte sensor elements in the analyte sensing devicesense different analytes.
 14. The method of claim 1, wherein a signalfrom an individual analyte sensor element within the plurality ofanalyte sensor elements adapted to contact and sense the analyte in theanalyte sensing device is individually interrogated.
 15. The method ofclaim 1, wherein at least one of the analyte sensor elements in theanalyte sensing device comprises a hydrogel disposed thereon, whereinupon exposure to an aqueous solution, the hydrogel expands in a mannerthat increases the permeability of the analyte sensor membrane.
 16. Amethod of extending the useful life of an analyte sensing devicecomprising analyte sensor elements that exhibit a decrease in theability to sense analyte over time due to biofouling or a loss ofactivity of an analyte sensing enzyme disposed on an analyte sensorelement; the method comprising sensing an analyte with an analytesensing device comprising: a plurality of analyte sensor elementsadapted to contact and sense analyte, the analyte sensor elementscomprising: a base layer; at least three working electrodes disposed onthe base layer; a glucose oxidase layer disposed upon the at least threeworking electrodes, wherein the least three working electrodes arecoated with the glucose oxidase layer so as to allow the analyte sensingdevice to sense glucose; an analyte modulating layer disposed on theglucose oxidase layer, wherein the analyte modulating layer comprises ahydrogel composition that includes a compound selected for its abilityto crosslink to a siloxane composition; an adhesion promoting layerdisposed between the glucose oxidase layer and the analyte modulatinglayer that functions to promote adhesion between the glucose oxidaselayer and the analyte modulating layer, wherein the adhesion promotinglayer comprises a siloxane composition crosslinked to the compound inthe analyte modulating layer and; at least one analyte sensor membranedisposed upon at least one sealable well in a manner that reversiblyprevents an analyte from contacting the at least one of the plurality ofanalyte sensors, wherein the permeability of the analyte sensor membranecan be controlled to allow an analyte to contact at least one of theplurality of analyte sensor elements; and at least one structureoperatively coupled to the analyte sensing device for relayinginformation away from the analyte sensing device; wherein the usefullife of an analyte sensing device is extended by: deactivating a firstanalyte sensor element in the plurality of analyte sensor elements thatcontact and sense analyte when the first analyte sensing elementexhibits a decrease in the ability to sense analyte due to biofouling ora loss of activity of an analyte sensing enzyme disposed on the firstanalyte sensor element; and activating a second analyte sensor elementin the plurality of analyte sensor elements that contact and senseanalyte by controlling the permeability of an analyte sensor membranedisposed upon the second analyte sensor element to allow an analyte tocontact the second analyte sensor element, so that the useful life ofthe analyte sensing device is extended.